UFO: Enemy Unknown (marketed as X-COM: UFO Defense in North America) is a science fiction strategy video game developed by Mythos Games and MicroProse. It was published by MicroProse in 1994 for DOS and Amiga computers and the Amiga CD32 console, and in 1995 for PlayStation. Its European PlayStation release is titled X-COM: Enemy Unknown.
Originally planned by Julian Gollop as a sequel to Mythos Games' 1988 Laser Squad, the game mixes real-time management simulation with turn-based tactics. The player takes the role of commander of X-COM – a clandestine, international paramilitary organization defending Earth from alien invasion. Through the game, the player is tasked with issuing orders to individual X-COM troops in a series of turn-based tactical missions. At strategic scale, the player directs the research and development of new technologies, builds and expands X-COM's bases, manages the organization's finances and personnel, and monitors and responds to UFO activity.
The game received strong reviews and was commercially successful, acquiring a cult following among strategy fans; several publications have listed UFO: Enemy Unknown as one of top video games ever made, including IGN ranking it as the best PC game of all time in 2000 and 2007. It was the first and best received entry in the X-COM series, and has directly inspired several similar games, including UFO: Aftermath, UFO: Alien Invasion, UFO: Extraterrestrials and Xenonauts. A remake of the game, XCOM: Enemy Unknown, was created by Firaxis Games and published by 2K Games in 2012.
The story of X-COM, set in the near-future at the time of the game's release, begins in the year 1998. The initial plot centers around increased reports of UFO sightings as tales of abductions and rumors of attacks by mysterious aliens become widespread. The nations of the world come to perceive this as a threat and attempt to form their own forces – such as Japan's Kiryu-Kai force – to deal with the crisis, but these efforts are unsuccessful. On December 11, 1998, representatives from some of the most powerful nations in the world secretly meet in Geneva to discuss the issue. From this meeting is born the clandestine defense and research organization Extraterrestrial Combat (X-COM), over which the player assumes control at the start of the game.
As the game progresses, the player learns more about the enemy, their species, mutated creations and technology. It is ultimately revealed that the aliens are led by creatures of immense mental power called Ethereals, and that their main base in the Solar System is located in Cydonia region of Mars. Then, the player must prepare the final assault team, attack Cydonia and destroy the mastermind behind the alien invasion, the biocomputer Alien Brain.
The game may end in several ways. If the player's performance is poor or worse for two consecutive months, the player runs a deep deficit for two consecutive months, all the player's bases are captured, or the player mounts an assault on the aliens' Mars base and loses, the game ends in defeat in which the council of funding nations make a futile attempt to negotiate with the aliens. If, however, the player is victorious in the final attack, the game ends in mankind's victory.